1. Field of the Invention
This invention generally relates to a hub transmission for a bicycle. More specifically, the present invention relates to a multi-speed hub transmission with a plurality of power transmission paths. The present invention further relates to a method for shifting such a hub transmission.
2. Background Information
Bicycling is becoming an increasingly more popular form of recreation as well as a means of transportation. Moreover, bicycling has become a very popular competitive sport for both amateurs and professionals. Whether the bicycle is used for recreation, transportation or competition, the bicycle industry is constantly improving the various components of the bicycle. Recently, the bicycle drive train has been extensively redesigned.
One type of a bicycle drive train utilizes an internally mounted multi-speed hub transmission for changing gears. Internally mounted multi-speed hub transmissions form part of the rear wheel of a bicycle. An internally mounted multi-speed hub transmission basically includes a hub axle that is mounted to the bicycle frame, a driver rotatably supported by the hub axle for receiving a pedalling force, and a hub shell rotatably supported by the hub axle. A power transmission mechanism is disposed between the driver and the hub shell transmitting rotational power from the driver to the hub shell through a plurality of power transmission paths. The hub transmission further comprises a shift mechanism that cooperates with the power transmission mechanism for selecting one of the pluralities of power transmission paths.
The power transmission mechanism typically includes a planetary gear mechanism with a plurality of sun gears, at least one ring gear and a planet gear carrier each of which is rotatably supported by the hub axle. The planet gears are rotatably supported by the planet gear carrier meshing with the sun gears and the ring gears. To change the power transmission paths and the corresponding gear ratios, the aforementioned various gear components are selectively non-rotatably coupled with each other by actuating the shift mechanism.
An example of this kind of a bicycle hub transmission is disclosed in European Patent Publication No. 1 323 627 A2. The hub transmission according to this publication is configured to provide for 8 speed stages. In speed stages 5 to 8, the driver is coupled with the planet gear carrier by a clutch, while in speed stages 1 to 4, the driver is uncoupled from the planet gear carrier. This change of the power transmission path when shifting between speed stages 4 and 5 is problematic in view of a desired shock-less shifting. To alleviate this problem, this publication proposes a particular timing of the coupling mechanisms that includes a shifting transition sequence. That is, when the bicycle is accelerating and the cyclist shifts from speed stage 4 to speed stage 5, the planetary gear mechanism is actuated to produce gear ratios corresponding to speed stages 3 and 7 prior to completing the shift to speed stage 5. Thereby, an undesired rapid acceleration of the pedals and a significant shock is avoided when the transmission completes the shift to speed stage 5.
When the bicycle is decelerating and the cyclist shifts from speed stage 5 to speed stage 4, a temporary transition from speed stage 5 to speed stage 7 and then to speed stage 3 prior to completing the shift to speed stage 4 is carried out to avoid a rapid deceleration of the pedals.
While the aforementioned shifting transition sequence reduces excessive acceleration or deceleration of the pedals when shifting between speed stages 4 and 5, the cyclist still feels a (reduced) shifting shock that still causes undesired discomfort.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved bicycle hub transmission. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.